The body's Humoral immune system is dominated by circulating Antibodies (proteins that bind to other
proteins specifically). These Abs are produced primarily by B Cell lymphocytes. The body's cell mediated immunity
involved T cells, macrophages, and other leukocytes.

All of the cells in the blood are derived from the same progenitor cells, the hematopoietic stem cells of the bone marrow.
Since we are dealing with the immune system, we shall be concerned only with cells of the leukocyte lineage (B-cells,
T-cells, and various monocytes such as macrophages). For an example of the development of a neutrophil, a component of
the innate immune system, take note of the image below. All of these cells begin their differentiation in the bone
marrow but, of the three cell types mentioned, only the B-cells complete it there. T-cells finish their development
in the thymus, and monocytes in the blood stream exit into the tissues where they become many different types of cells
(eg. macrophages and dendritic cells).

However, even after maturation lymphocytes(B and T cells) have yet to undergo many changes
before they are ready to carry
out their duties as the mediators of the adaptive immune response. T
Lymphocytes come in two main varieties: Cytotoxic T-Cells and Helper
T-cells. Each plays an important role in effectively controlling
infections and aiding the humoral side of the immune response to an Ag.
When an antigen is first encountered, typically a Macrophage engulfs it
and breaks it apart using very acidic conditions and proteases. Next,
the Macrophage will present a small portion of this antigen (6-20
Amino Acids) on its surface. This Antigen is presented with an MHC Type
II complex (Major Histocompatibility Complex)- used exclusively in the
cells of the immune system. If a non-immune cell displays an antigen, it
is typically with an MHC type one complex (indicating to Cytotoxic
T-cells that they should destroy this presenting cell). The cell is
destroyed because if something is presented with MHC I, it signals that
the Antigen is inside the cell (e.g. viral genome products). If a cell is
infected, there is little your immune system can do but kill it to
prevent viral spread. A typical presentation is that by a Macrophage to a Helper T-cell.
The Helper T-cell will process
the antigen and then signal to a B Lymphocyte to product Antibodies to the Antigen.
Ciruclating antibodies are perhaps the most important way to keep foreign material out
out of your body. Antibodies also give you the ability to "remember" an antign and properly
erspond to it if you encounter it again.